For though by this time you ought to be teachers, you need someone to teach you again the first principles from the oracles of God; and you have come to need milk and not solid food. For everyone who partakes only of milk is unskilled in the word of righteousness, for he is a babe. But solid food belongs to those who are of full age, that is, those who by reason of use have their senses exercised to discern both good and evil.
One does not need to attend a parenting class to know that the food one feeds a new baby is much different than the food older children and adults eat. Imagine trying to feed a steak to a newborn babe. Not only is that child’s digestive system incapable of processing that meat, but a baby has no teeth to chew it. That solid food is not what the child needs at that time. Conversely, one would not feed only milk or formula to a child through the age of six or 12 or 17. That would be an equally grave disservice because that child would not receive the nutrition necessary to grow out of infancy. Naturally, there is a time when milk suffices and a time when solid food must step in to do the job.
Paul is admonishing some in the Church who have not met the expectation of moving from milk to solid food. He is telling them that they have not grown in their discernment of what is good and what is evil, which should occur naturally when we are practicing our spiritual discipline. This spiritual milk we drink, the basic principles that serve to prop up more complex tenets, is good for a time. We know to move on from that milk when we have grown all we can from it, and we cannot move on to solid spiritual food until that milk has transformed us as it should. However, if we decide to feed only on spiritual milk yet not grow from it, we never will be ready for solid food, and we never will grow as God wants and needs us to grow. He did not intend that his children would remain spiritual infants.
Just like the natural child, spiritual milk is good for a time, but growth is necessary for the solid food to be introduced. The difference between our natural growth and our spiritual growth is that spiritual growth is a choice. We can decide to stick with the milk, but that will mean that we only regress. Spiritual maturity is a natural consequence of learning and growing from that milk, but it starts as a choice to desire the solid food which lay ahead. Yet, if we move to solid food before we able to digest it, it will not do us good. This is about desire, discernment and timing. Father, keep us submissive and softhearted to the teaching your word brings so that we are able to move from milk to solid food and grow as you need us to grow.